The acquired immunodeficiency syndrome (AIDS) has become a global pandemic with over 260,000 cases reported from 156 countries. A major focus within our laboratory has been on defining the unique epidemiologic, clinical, virologic and immunologic features of HIV-1 infection in Africa and other tropical countries. In a survey of 12,032 employees and their spouses at two large businesses in Kinshasa, Zaire, 4.5% were seropositive for HIV-1 with a 0.5% annual seroincidence predominately due to heterosexual transmission. In this study, AIDS continued to be the leading cause of death, among employed individuals responsible for 50% of deaths. We also demonstrated that 40% of 1200 female prostitutes in Kinshasa were HIV-1 seropositive and that incident infections (3%/yr) were strongly correlated with genital ulcers and chlamydial infections. In studies of perinatal infection in Zaire, Kenya and Haiti, we documented a 25-35% transmission rate of HIV-1 infection and a cumulative mortality rate of 70% in the first two years of life. Of surviving children during the first year of life a beneficial effect of immunizations against commonly occurring childhood infectious diseases was observed despite the lower immunogenicity in HIV-1 seropositive individuals. In virologic studies we demonstrated the presence of HIV-1 genome by PCR and/or culture in 5% of 101 seronegative high risk individuals confirming a prolonged latency period before seroconversion. HIV-1 antigenemia was also observed to be decreased in Africans and U.S. blacks, probably secondary to p24 antibody-antigen complexes secondary to increased p24 antibody levels. Additional studies demonstrated the presence of neutralizing antibody to the V3 loop region of HIV-1(mn) in HIV-1 infected individuals in the U.S., Zaire and Brazil suggesting the importance of this particular epitope of HIV-1(mn). Serologic screening for HTLV-I/ll demonstrated increasing rates of HTLV-L infection among Africans and blood transfusion recipients in the U.S., and HTLV-II among IV drug users. Further studies will continue to examine the immunopathogenesis and natural history of these human retroviruses in selected populations.